This invention relates to misfire detection devices for detecting misfires of internal combustion engines, and more particularly to misfire detection devices for detecting the misfires occurring continuously in the cylinders of multi-cylinder internal combustion engines.
A conventional misfire detection device for an internal combustion engine is disclosed, for example, in Japanese Laid-Open Patent (Kokai) No. 2-414000. The misfire detection device detects the rotation of the internal combustion engine by means of a crank angle sensor, and determines an occurrence of misfire on the basis of the variation in the rotational speed of the internal combustion engine caused by the misfire. Namely, the ratio of the times required by the rotation of the crankshaft to complete predetermined angles before and after a predetermined crank angle is determined. Further, the acceleration of the time ratio is determined and an occurrence of misfire is detected when the acceleration of the time ratio exceeds a predetermined level.
According to the above misfire detection device, the misfire is detected on the basis of the acceleration of the time ratio corresponding to a single cylinder. Thus, if the state of the misfire changes and the cylinder is normally operating in a first cycle and misfiring in a second cycle, the acceleration obtained for the first cycle and that obtained for the second cycle are distinct, and hence the occurrence of misfire can be detected reliably. If, however, the cylinder is continuously misfiring in both the first and second cycles, the difference between the acceleration becomes negligibly small, such that the detection of the occurrence of misfire becomes difficult. The detection of occurrences of misfire may thus become inaccurate and unreliable.